When is a player in an offside position actually penalised?

Only if, at the moment a team-mate plays or touches the ball, the player is in an offside position and becomes involved in active play (Law 11.2, 2026/27) by:

• interfering with play: playing or touching the ball passed or touched by a team-mate
• interfering with an opponent, by:
  • clearly blocking the opponent's line of vision
  • challenging the opponent for the ball
  • clearly attempting to play a nearby ball when this affects an opponent
  • making an obvious action that clearly affects an opponent's ability to play the ball
• gaining an advantage by playing the ball or interfering with an opponent after it rebounds or deflects off the post, crossbar, a match official or an opponent, or after a deliberate save by an opponent

Not offside:

• receiving the ball directly from a goal kick, throw-in or corner kick
• receiving the ball from an opponent who deliberately played it (for example a misplaced pass or clearance), except a deliberate save

The restart is an indirect free kick to the defending team where the offence happened.